1. Scope of the Invention
The present invention relates to a phase lock circuit for the recognition of digital information read out from a movable magnetic media, typically for disk units.
2. Description of the Prior Art
The magnetic recording technique commonly used in such units is the modified frequency modulation (MFM).
The binary information is recorded along each track of the magnetic media in sequence as magnetic transitions located in cells, which, in relationship to the speed of the media and a read/write head, are defined by a time interval or nominal duration.
For a typical rigid disk unit having a ST506 interface the cell has a standard nominal duration of 200 nanoseconds (ns).
For a 51/4" flexible disk unit the cell has a nominal standard duration of 4 microseconds (usec).
Recently 51/4" flexible disk units have been put on the market with nominal cell duration of 2 usec.
In addition units for flexible disks having 8" diameters continue to be used in the market with a nominal cell duration of 2 usec (recorded in MFM or double density) and with a nominal cell duration of 4 usec (recorded in FM or single density).
In the MFM magnetic recording, information bits having a logic level 1 are recorded as a magnetic transition at the center of the cells and bits having a logic level 0 are recorded as having no transition in the cells.
In order to provide a timing signal even in presence of a "zeroes" sequence, subsequent "zero" cells are identified by a magnetic transition recorded at the boundary between the cells.
This transition is missing if a cell containing a bit "0" is preceded or followed by a cell containing a bit "1".
The FM coding method, contemplates the recording of a magnetic transition at each cell boundary and the presence of a transition in the middle of a cell in case of a "1" or no transition in the middle of a cell in case of a "0".
When the magnetic media is read, each transition generates an electrical read pulse.
Therefore, in case of cells having a nominal duration of 200 ns the read pulses nominally occur 200, 300, or 400 nsec apart. In the case of cells having nominal duration of 2 usec, the read pulses nominally occur 2, 3, or 4 usec apart. In the case of cells having a nominal period 4 usec, the read pulses nominally occur 4, 6 or 8 usec apart. In the case of cells having a nominal period 4 usec (but with FM or single density coding) the read pulses nominally occur 2 or 4 usec apart.
Owing to changes in the rotational speed of the media and to the so called "peak shift" phenomena of the recorded magnetic pulses, the effective durations of the read out intervals differ from the nominal durations and give rise to a signal having a variable phase and frequency which makes the discrimination among bits 0, 1 and timing pulses particularly difficult.
Phase lock circuits are used to perform such discrimination. These are circuits which, in response to the sequence of electrical pulses produced by reading the disk, generate a timing signal having a frequency which varies from the basic frequency of the read out pulse sequence, but which ignore the significant modulation due to the magnetic recording process.
Such circuits must lock in a fast way to the read out data frequency, must be able to quickly follow the magnetic media speed changes, and must ignore the phase changes due to the significant modulation.
In addition, such circuits should operate at different working frequencies, in order to work with media recorded at different frequencies and in different coding modes.
Phase comparators, in form of integrated circuits have been recently put on the market.
By adding some external components to the phase comparators, it is possible to obtain discriminating phase lock circuits. However such circuits have limitations which prevents the design of reliable discriminators for a wide range of the magnetic media speed variation, which have fast locking and are stable in operation.
The phase lock circuit which is the object of the present invention overcomes such limitations and is further capable of handling magnetic media recorded in single or double density at different frequencies, typically the standard frequencies of 5 MHz, 500 KHz and 250 KHz.